This invention relates to vibrating screens.
The aggregate industry utilizes many styles of screen machines to sort aggregates by size. Most screen machines utilize vibration to agitate the mixture of aggregates to promote separation through various sized openings in the screening surfaces. Sorting is achieved by undersized particles passing through the openings in the screening surface and the oversized particles being retained above the screen surface. These machines usually have some type of vibrating mechanism to shake the unit and its screening surfaces. The vibrating mechanisms usually include an unbalanced weight mounted on one, or several, rotating shafts which, when rotated, force a cycling motion into the screen machine.
Sometimes a screen is designed with several layers, or decks, of screening surfaces which have screen media of various sized openings to allow sorting of granular material, which is fed into the machine, into several discreet particle sizes. These layers may be herein referred to as decks or screens.
The screen surface media normally consists of a wire mesh or flexible panel with punched or formed holes, all which have specific sized openings to allow passage of sized particles to the decks below, or out the bottom of the screen. The larger sized particles are retained above the surface and are usually discharged on the end opposite the feed end of the deck.
The screen media is normally sized with larger holes in the upper decks and smaller holes in the lower decks. A mixture of granular material, comprised of a variety of sized particles, is fed onto the top deck, which normally has the largest holes. Material smaller than the holes then falls through to the next level, while the material larger than the holes is retained on the deck. The material that has fallen through the holes settles onto the next lower deck. The next lower deck normally has smaller holes than the deck directly above. The material that is smaller than the hole falls through this deck while the material larger than the hole is retained, thus leaving a very specific size of material on this deck, smaller than the deck holes above, larger than the deck holes below. This is then repeated on lower decks depending on how many decks are employed in the screen machine. There can be many deck levels depending on how many different sized materials are desired from the machine.
For a continuous screening machine, the motion of the screen normally propels the material from one end of the screen known as the feed end, toward the opposite end known as the discharge end. Material can be continuously fed onto the feed end of the top most deck and as it flows across and down through the decks, various sized material are ejected from the discharge end of each sizing deck.
As the material travels down the decks, and until the undersized material (smaller than the holes) falls through the holes, there is some lag time until the particles can align and fall through the holes. The lag time before material starts hitting the lower deck reduces the effective screening surface of the lower deck. The industry normally assumes a lag time effect, i.e. an approximate reduction of 10% of the screening surface per deck level when computing the theoretical capacity of passing material through a deck. For example, if a top deck is 4′ wide and 10′ long from feed end to discharge end, the effective size is 4×10=40 square feet of screen surface on that deck. The next lower deck, assuming 10% reduction attributable to the lag time effect, the effective screen surface on this deck is (1−0.1)×4×10=36 square feet. Again, for a third deck, the effective screen area is (1−0.1−0.1)×4×10=32 square feet.
Consequently, there is a need for improvement in sorting systems for multi-deck vibrating screens.